1. Field of the Invention
The present invention relates to a two-stage supercharging system for an internal combustion engine provided with a high-pressure-stage turbocharger and low-pressure-stage turbocharger capable of finely controlling an EGR rate and air-fuel ratio.
2. Description of the Related Art
As for supercharging of the internal combustion engine, for the purpose of expanding an operation region of an exhaust turbocharger, waste gate turbocharger, variable capacity turbocharger, and so on have been developed. However, when trying to obtain a high pressure ratio (high supercharging pressure) with one supercharger in a wide range of rotation region of the internal combustion engine, since operation efficiency is lowered because of characteristics of turbochargers and the range of operation flow rate reduces because a surge is easy to occur, there is a limit to the high pressure ratio.
Therefore, for the purpose of obtaining high supercharging pressure by multi-stage pressure rising and operation efficiency improving of the supercharger, the two-stage supercharging system is used. In the two-stage supercharging system, two superchargers are serially arranged in two stages and pressure is risen by dividing intake air into two stages, low-pressure-stage and high-pressure-stage. Thus, by reducing rising pressure ratio at each single stage, operation efficiency is improved.
One of the two-stage supercharging systems is a series type two-stage supercharging system. In this system, basically it is assumed that both low-pressure-stage and high-pressure-stage should be simultaneously used and no operation control is required for the high-pressure-stage exhaust turbine by the flow rate control of the exhaust gas.
When combining a reciprocation-type internal combustion engine having a wide operation range with an exhaust turbocharger having a narrow operation range in the system, since effective supercharging is limited to a special operation region because of the characteristics of the both, an exhaust turbine is matched by adjusting the operation region to the output point of the internal combustion engine.
However, there is a problem with the system that when the operation condition of the internal combustion engine is under low-speed and low-load region, exhaust pressure and exhaust flow rate is not enough. In addition, there are other problems that since the capacity of the turbocharger is too large to sufficiently function, high supercharging pressure (high boost) cannot be obtained or response delay may be increased.
Another two-stage supercharging system is a sequential-type supercharging system. In the system two turbochargers having extremely different capacities to each together are serially arranged and a bypass is provided so that a suitable supercharger can be selected according to operation conditions of the internal combustion engine. Thereby, operation region as a supercharging system is expanded.
Unlike the series-type two-stage supercharging system, when in a high-speed rotation and high-load region, the system uses a large capacity low-pressure-stage turbocharger suitable for the operation region to perform supercharging, which is the same or more than the single-stage supercharging system of conventional techniques. On the other hand, in the low-speed to medium-speed rotation, using a small-capacity high-pressure-stage turbocharger instead of low-pressure-stage turbocharger, supercharging is performed.
With this system, by making the capacities between low-pressure-stage and high-pressure-stage turbochargers have a large difference, it becomes possible to supercharge even in a difficult-to-supercharging operation region for the series-type two-stage supercharging system.
In the system under a full load operation, when a small-capacity high-pressure-stage turbine is operated, the exhaust pressure increases. Therefore, under some operation conditions, all exhaust gas has to be made flow into the low-pressure-stage turbine by bypassing the high-pressure-stage turbine. As a result, according to operation conditions, low-pressure-stage and high-pressure-stage turbochargers are switched for use.
By using a small flow rate high-pressure-stage turbo under a low-speed and low-load operation, the system realizes high supercharging and high EGR rate, and further realizes low NOx and low soot combustion. Moreover, since high response is obtained by the turbine inertia becoming smaller, there is no turbo lag at the time of acceleration, so that drivability is also improved. Under a high-speed and high-load operation, by switching to the low-pressure-stage turbo having the same size as the sequential type, deterioration of engine performance can be avoided.
However, when operation conditions of the internal combustion engine is in a low-speed rotation region, all exhaust gas has to be sent to the high-pressure-stage turbine in order to obtain a required supercharging pressure. When in a medium-speed rotation region, since supercharging is conducted using both low-pressure-stage and high-pressure-stage turbochargers, the exhaust gas flow rate sent to both high-pressure-stage and low-pressure-stage turbines has to be precisely controlled.
Therefore, in this system, a high-pressure-stage exhaust bypass passage and a bypass valve for flow rate control therein are provided. The bypass valve is required such that while under the operation condition that the exhaust gas flow rate is large, the valve has flow rate characteristics that all exhaust gas can be flown without causing an increase of the exhaust pressure, the exhaust gas flow of the high-pressure-stage turbine can be controlled under the small to medium flow rate operation condition of the exhaust gas.
However, since high temperature exhaust gas as high as 800° C. flows in the high-pressure-stage exhaust bypass passage, production of a bypass valve for flow rate control is technically very difficult, which can secure circulation of a great flow amount of exhaust gas at the time of high-speed and high-load operation and at the same time can control small amount of gas at the time of low-speed and low-load operation. Consequently, there is a problem that in the sequential type two-stage supercharging system, it is difficult to precisely control the flow rate of the exhaust gas flowing into the high-pressure-stage turbine to conduct a suitable supercharging in each operation condition according to operation conditions of the internal combustion engine.
As a countermeasure for it, for example, a supercharging pressure control device having a two-stage turbo supercharging engine is proposed like the description of Japanese Patent Application Kokai Publication No. S61-277818. With the device, in the two-stage turbo supercharging engine, two bypass valves, primary and secondary, having different diameters of valves are provided in parallel in the exhaust bypass passage detouring the exhaust turbine of the high-pressure-stage turbocharger. As for the secondary bypass valve having a small diameter, valve opening is adjusted and controlled according to the change of output pressure of the high-pressure-stage compressor in the low- and medium-speed range of the engine with its pressure being a target. At the same time, after the output pressure of the low-pressure-stage compressor reaches a target in the high-speed region of the engine, the primary bypass valve having a large diameter immediately fully opens to be switched into a single-stage supercharging by the low-pressure-stage turbocharger.
With the configuration above, the following effects are assumed to be produced. Since the pressure can be precisely adjusted to a target by a minute opening and closing operation of the secondary bypass valve, pressure fluctuation by such as hunting can be prevented like pressure regulating operation by the main bypass valve with a large diameter. When the output pressure of the low-pressure-stage compressor reaches a target, the primary bypass valve is immediately moved to the full-open position to be switched to a single stage supercharging. Thus, a suitable role is taken for primary and secondary valves, respectively, so that regulation of pressure and switching operation can be smoothly and securely conducted.
However, in the supercharging pressure control device of the two-stage turbo supercharging engine, suitable dimensions are not shown for a large-diameter primary bypass valve and small-diameter secondary bypass valve, so that it is not possible to perform practical flow rate control of the exhaust gas only by the information above.
For example, in the two-stage turbo supercharging engine, when performing EGR, air-fuel ratio (A/F) and EGR rate are controlled within a small range of equal to or less than 0.1 and 1%, respectively. As a result, there occurs a problem that when configuring a valve opening effective area necessary for making maximum exhaust gas flow rate bypass at full load without any difficulty with an appropriate ratio, control precision becomes poor in an infinitesimal angle region, so that fine EGR rate control and air-fuel ratio control cannot be conducted.    Patent document 1: Japanese Patent application Kokai Publication No. S61-277818